The present invention relates to a tool for fine machining of optically active surfaces such as is used for example in lens production in fine machining of optical lenses. In particular the invention relates to a tool for fine machining of free form surfaces and toric surfaces on spectacle lenses.
When the description below uses the term “spectacle lenses” as an example of optical workpieces, these refer not only to spectacle lenses of mineral glass but also to spectacle lenses of all other conventional materials e.g. polycarbonate, CR 39, HI index, etc. i.e. also plastics.
Cutting machining of optically active surfaces of spectacle lenses can be roughly divided into two phases, namely first the pre-machining of the optically active surface to generate the prescription macro-geometry, and then the fine machining of the optical active surface to remove the traces of pre-machining and obtain the desired micro-geometry. Whereas pre-machining of the optically active surfaces of spectacle lenses, depending amongst others on the material of the spectacle lenses, takes place by grinding, milling and/or turning, the optically active surfaces of spectacle lenses in fine machining are usually subjected to a fine grinding, lapping and/or polishing process. Mainly rigid forming tools are used here which serve as a support for fine grinding films or polishing compound carriers.
The prior art has repeatedly found (e.g. DE 44 42 181 C1, EP 0 884 135 B1, DE 101 06 007 A1) that a disadvantage of rigid forming tools is that a large number of forming tools is required to fine machine the multiplicity of possible lens geometries occurring in prescription production of spectacle lenses (convex or concave curvatures from 0 to 17 diopters, where applicable with cylinder effect with up to 6 diopters) with possibly deviating refractive indices of the various materials. Further difficulties occur in the trend for spectacles to use increasingly multi-focal lenses in the form of progressive focal lenses in which the distance vision area transforms progressively into the near vision area. At least one of the optically active surfaces of such spectacle lenses has an individually designed macro-geometry which also has free form surfaces and naturally must also be fine machined.
In the prior art there is no lack of suggestions of how tools should be designed for fine machining of optically active surfaces in order to cover as wide as possible a range of geometries with one tool.
In this context DE 44 42 181 C1 by the applicant discloses a tool for fine machining of optical surfaces of lenses, with an elastic membrane having a machining section which is attached via a fixing section to a rigid holder. The rigid holder together with the elastic membrane delimits a cavity filled with a filling material which, as a mass deformable plastically under certain conditions, forms optionally under control a flexible or rigid supporting layer for the membrane so that before the start of the fine machining the outer contour of the membrane can be adapted to the form of the optical surface. According to this prior art the membrane furthermore has between its machining section and its fixing section a gaiter-like section which, on contact of the machining section with the optical surface, applies forces to the plastically deformable mass so that this presses the machining section onto the optical surface so that the tool retains its shape after hardening of the plastically deformable mass. Using this tool a relatively wide range of lens geometries can be fine machined. The softening and subsequent hardening of the plastically deformable mass however requires some time, so that this tool can only be used with restrictions in industrial production of prescription lenses.
In other solutions (EP 0 884 135 B1, DE 101 06 007 A1) in which the tool is also able to form the surface to be fine machined before this machining, the tool has two axially spaced elastic membranes held on a base body, between which is provided a multiplicity of pins which can be moved in the longitudinal direction by pneumatic action on the membrane inside the tool in order to adapt the membrane outside the tool to the surface to be fine machined. When this adaptation is made, the pins are fixed to each other pneumatically or magnetically in the transverse direction in order to form a rigid machining surface on the tool. One particular problem with these tools however is that these tools are very complex in design and consequently susceptible to fault.
Finally tools for polishing optical surfaces are also known (DE 101 00 860 A1, DE 101 06 659 A1) which have a rigid base body that is connected articulated and rotationally rigid with the tool spindle and on the machining side carries an elastic intermediate layer and on this the polishing layer itself. These tools can naturally only be adapted to the optical surface to be fine machined insofar as the elastic intermediate layer permits it which, to ensure control of the polishing process, cannot be formed arbitrarily thick.
To summarize it can be found that there is still a need for an adaptable tool for fine machining especially of spectacle lenses which can be used as universally as possible in the industrial production of prescription spectacle lenses, is economic and at the same time reliably guarantees good machining results.